Flow rates in pipeworks and passages can be determined by means of ultrasound measurement technology using the transit time difference method. An important and demanding area of application is represented by gas meters for natural gas pipelines where, due to the huge gas volumes conveyed and to the value of the resource, even the smallest deviations in the measurement precision correspond to very noticeable economic values.
A known measurement principle is shown in FIG. 5. As essential components of a conventional measurement apparatus 110, two ultrasonic transducers 118, 120 are arranged at an angle in the wall of a conduit 112 in which a fluid 114 flows in the direction of the arrow 116. Ultrasonic pulses are output and received transversely to the flow of the fluid on the measurement path between the ultrasonic transducers 118, 120, with the ultrasonic transducers 118, 120 operating alternately as transmitter and receiver. The ultrasonic signals transported through the fluid are accelerated in the direction of flow and are decelerated against the direction of flow. The resulting transit time difference is calculated using geometrical parameters to form a mean flow rate of the fluid. Together with the cross-sectional area, the operating volume flow results from this which is the measurement variable of interest for a fluid billed by volume, for example. The geometrical relationships are described by the following variables:
v: flow rate of the fluid in the line
L: length of the measurement path between the two ultrasonic transducers
α: angle at which the ultrasonic transducers transmit and receive
Q: volume flow
D: diameter of the line
tv: transit time of the ultrasound with the flow and
tr: transit time of the ultrasound against the flow
The following relationships result from this for the sought variables v and Q:v=L/(2 cos α)(1/tv−1/tr)andQ=v¼D2π
The local, mean flow speed at the position of the measurement path is accordingly determined in this manner. However, this only produces an accurate measured value with uniform flows. A plurality of measurement paths are therefore geometrically distributed over the cross-section of the conduit for demanding applications. A more precise value for the mean flow rate is then determined over the total cross-sectional area by a weighted addition of the measured values of the individual measurement paths. A series of measurement path configurations or layouts are presented in the standard ISO 17089-1.
Ultrasound measurement apparatus are known that are made up of a plurality of subsystem each having one or more measurement paths. This reduces the complexity in the individual subsystems and provides redundancy. However, signal interference can occur that impairs the quality of the received ultrasound signals.
Such a measurement apparatus is known from EP 2 310 808 B1. It comprises a plurality of transducer pairs that are operated in two groups of a respective one set of control electronics. In this respect, the two control electronics are coupled to one another communicationally and coordinate the activity of their transducer pairs such that the two subsystems are never simultaneously active and thus mutual interference of ultrasound measurements on the respective measurement paths are precluded.
This communication and synchronization admittedly solves the problem of signal interference, but has the consequence that always only one subsystem can make a measurement. A strictly sequential operating regime thus results that only utilizes half the measurement time per subsystem. The redundancy of the systems is furthermore lost since in the event of defect or of other errors in a subsystem, the loss of the communication or of the synchronization is also accompanied by a loss of the required coordination and the operational reliability is thus called into question.